The major goal of this project is the definition of mechanisms of immune recognition of viral antigens on the cell surface of virus infected cells. We are continuing to study the murine leukemia line FBL-3 which undergoes a major phenotypic alteration in vivo converting from benign to malignant behavior. Cells grown in vitro or for 7 days in the ascites from produce transient tumors when injected subcutaneously. The cells passed in ascites for 14 days produce lethal subcutaneous tumors which metastasize. We have found by fluctuation analysis that malignant tumor cell variants exist in the parent line and that as yet undefined selective pressure in the host allows the preferential expansion of the pre-existing variants. Detailed studies of the surface antigens expressed by cloned tumor cell variants have revealed important aberrations in the major histocompatibility antigens expressed by these cells which may adversely affect their immune recognition by the syngeneic host.